The invention relates to a method of coating a metallic substrate with thermoplastic coating material.
In connection with this application, thermoplastic materials must also be understood to mean plastic materials which are composed essentially of thermoplastics and furthermore of additives which are added to obtain particular properties.
The coating of metallic substrates with plastic layers is known in practice and is suitable, in particular, where the metal used has to be protected against atmospheric, generally corrosive, conditions or where the environment has to remain safeguarded from undesirable action of the metal or where both the abovementioned effects have to be avoided, such as in the case of packagings. Certain foodstuff cans, for example, have long been manufactured from tinned packaging steel, also called tinplate, provided on one side with a plastic layer in order to prevent the metal from entering the foodstuff and from rusting through occurring as a consequence of corrosive action of the foodstuff and, on the other hand, to preserve the fine appearance of the packaging for a sufficiently long time.
These coating layers, which are usually organic, are generally applied by lacquering. It may be necessary to apply different covering layers for the inside and outside of the lid. The covering layer on the inside must primarily offer a good corrosion protection against the respective product to be preserved and to be packaged, while, on the outside, the appearance, for example, the gloss and the colour, and mechanical resistance to scratching and impacts are of importance.
A machining step in the creation formation of a packaging, such as the folding of body and lid in the case of a foodstuff can or drink can, may give rise, for example, to the imposition of more detailed requirements on the covering layer. It is of importance that the covering layer on the outside is both scratch-resistant and readily deformable so that the folding operation is withstood without damage.
To apply an organic top coat, apart from lacquering of the partly finished packaging, it is also conceivable to carry out the coating at an earlier stage in the production chain in the form of a coil-coating process, for example coil lacquering, foil film coating (also referred to as coil laminating), in which a plastic film is applied to a substrate, and extrusion coating, a form of film coating in which the film is not supplied from elsewhere, but, as it were, is produced by extrusion at the point which is being coated and applied to a substrate.
Any method of coating has its own disadvantages, be it in the nature of the coating material or in the nature of the manner of coating. It is clear that the drawback of evaporation of solvents applies to lacquering, which is even a very great drawback if the solvents contain VOCs (=volatile organic compounds), which is the case for many applications.
Film coating is generally not regarded as an economically feasible alternative. The production of the film, rolling it up, transporting it and unrolling it again result, when taken together, in an alternative which does not compete with lacquering.
The extrusion coating process as a method of application is attractive because it does not have the abovementioned economic disadvantage, but it proves difficult to carry out in the case of certain combinations. This applies mainly to single-sided or double-sided coating with polyolefins. Said polyolefins do not inherently have the properties which are necessary for good adhesion to metal substrates. For this reason polyolefin coating layers are usually chosen which are provided with a modified adhesion layer. Said adhesion layer is then composed of polyolefins modified with carboxylic acid groups or anhydride groups. Suitable adhesion layers based on polyolefins can be produced, for example, by copolymerization of propene or ethene or a combination of the last two with xcex1-unsaturated carboxylic acids, xcex2-unsaturated carboxylic acids, the associated anhydrides or associated esters or half-esters. Examples of these are acrylic acid, methacrylic acid, crotonic acid, cinnamic acid, maleic acid, maleic anhydride, fumaric acid and the like. The percentage by weight of such modifying monomers is normally between 1 and 15%.
Such adhesion-promoting monomers are capable of binding to the substrate surface but do so only if the polyolefin adhesion layer is at a temperature above the melting point for some time, as a result of which diffusion of the adhesion-promoting monomers through the polyolefin to the substrate surface becomes possible and the adhesion-promoting monomer may react chemically or physically with the substrate surface. It is found to be impossible to bring about adequate adhesion for further transportation of the product through the line by means of extrusion of a polyolefin provided with adhesion-promoting monomers onto a metal substrate which is brought to a temperature which is below the melting point of the respective polyolefin. Instead of that, the film follows the track of the internally cooled metal pressure roller which presses the coating layer onto the substrate and the film is rolled up onto the pressure roller. Preheating the metal strip to a temperature above the melting point of the polyolefin is, of course, very readily possible. This results in an adhesion between the modified polyolefin coating layer and the metal strip. However, if the other side of the strip has also to be coated and this does not take place at the same time as the coating of the abovementioned side, the problem arises that the coating layer of the abovementioned side is damaged during the step of coating the other side because the temperature of the product is, after all, above the melting point of the polyolefin coating layer. As a result, the coating layer comes into contact with the steel pressure roller in the molten and, consequently, very fragile state and is damaged.
In regard to the modification of the plastic film, such as, for example, by means of oxidation of the surface by a corona treatment, extrusion at very high temperature or a flame treatment, it may be pointed out that these are generally not readily feasible for the in situ modification of a molten, very thin sheet of plastic. A corona treatment ensures the production of an electrostatic charge, as a result of which the molten sheet is repelled from the corona installation and the treatment loses its effectiveness. In the case of extrusion of the polyolefin at very high temperature, the free surface of the coating layer oxidizes, resulting in the packaging material having an increased effect on the taste of the filling agent; a flame treatment is accompanied by a gas flow which repels the molten sheet of plastic and, in addition, develops a fairly large amount of heat around the mouth of the gun head.
Surprisingly, modification can in fact be carried out successfully by making use of a very small amount of ozone gas, which oxidizes the surface of the sheet of molten polyolefin in such a way that some adhesion to a substrate which is at a preheating temperature lower than the melting point of the polyolefin is produced even in the laminating nip. This in-situ adhesion is found to be adequate for the coating layer to follow the substrate and, consequently, not the internally cooled pressure roller, to the afterheating section, where further adhesion improvement takes place by afterheating to a temperature above the melting point of the polyolefin, followed by diffusion of the adhesion-promoting groups to the substrate surface and further chemical or physical reactions.
It is furthermore found that copolymerization of propene with ethene has the result that the action of the ozone gas is improved, as a result of which the initial adhesion of the coating layer to the substrate increases.